Make print() use a single write() call

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python
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backend

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The builtin print() lives in Python/bltinmodule.c (builtin_print_impl), which currently issues separate writes for each argument, the separator and end. Read that function first, then the existing tests in Lib/test/test_builtin.py covering print to see the expected partial-output and flush behaviour. Done means one buffered write per call, partial output preserved when str() raises, plus tests for the atomicity and error paths.

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描述

interpreter-core topic-IO type-feature
Feature or enhancement

Currently, print("a", "b") performs four separate writes:

file.write("a")
file.write(" ")
file.write("b")
file.write("\n")

The proposed behavior is equivalent to:

file.write("a b\n")

The main motivation is to reduce interleaving between concurrent print() calls. For example, two threads printing separate messages can currently produce msg1msg2\n\n. Atomicity would still depend on the underlying stream.

Reducing the number of calls can also improve performance, particularly for streams with expensive write() methods, such as IDLE’s output stream.

The implementation should preserve partial output when converting an argument with str() raises: write the parts collected so far, without appending end, and propagate the exception. If flush=True, flush that partial output as well.

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

https://discuss.python.org/t/make-print-use-a-single-write-call/109886

Linked PRs
  • gh-161936
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